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Thanks i see what you mean, it would be handy to have the frequency filtering in there...but to be honest i suspect we may be able to get away without that, as usually the place is quiet...(well, no continuously loud noise, just the occasional loud bang)......and to filter down to 1kHz to 4kHz is going to need a relatively big filter, so  i am not so sure if our cheap-as-chips mindset can extend to that. Thought to use a cheap opamp as will need amplification.

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Hi,

Please advise if you know of a cheaper/simple way to detect the continuous

tone at 1khz to 4khz which comes out of the machine continuously

when its failed? The three option circuits attached (A,B,C) seem to do the job. (LTspice and .PNG attached)

However, as betwixt kindly says, it would be nice to get some bit of frequency

discrimination in there aswell if possible.

I used a piezo disc microphone for cheapness. (thanks to betwixt for putting it forward)

Please entertain the most crude-as-you-like circuits of simplicity as yet untold,

for us, where "yeah, does the job" is far more important than "quality way of doing it".